Plug cap device for internal combustion engine

Abstract: There is disclosed a plug cap device (11) for an internal combustion engine wherein an opening (15) receiving a corrugation portion and a cylindrical portion of an insulator of a spark plug has a first portion (15a) corresponding to the insertion position of the corrugation portion and a second portion (15b) corresponding to the insertion position of the cylindrical portion such that the inner diameter (P) of the first portion (15a) corresponding to the insertion position of the corrugation portion of the insulator is slightly smaller than the outer diameter of a major diameter portion of the corrugation portion and such that the inner diameter (Q) of the second portion (15b) corresponding to the insertion position of the cylindrical portion of the insulator gradually decreases from the inner diameter (P) of the first portion (15a) toward an insertion end at which the spark plug is inserted to provide a tapered configuration, whereby the plug cap device (11) insures an improved electrical sealing property and provides high operating efficiency when fitted over and removed from the spark plug.

Description: The present invention relates to a plug cap device for an internal combustion engine for connection to a spark plug used in the internal combustion engine. A known plug cap device the present invention starts from is known from US-A-3,995,929. Other known plug cap devices for carrying high voltage to a spark plug in an internal combustion engine are disclosed in e.g. Japanese Utility Model Application Laid-Open No. 64-8580 (1989) and Japanese Patent Application Laid-Open No. 3-264776 (1991). As shown in Figs. 5 and 6, for example, a known plug cap device 1 comprises a tubular plug cap body 2, an ignition cable 3 introduced into the plug cap body 2 at its one end, and an electric conductor 4 housed and held in the plug cap body 2 and connected to the ignition cable 3. The plug cap body 2 is a flexible insulator formed of an elastic material such as silicone rubber. The plug cap body 2 is press fitted at its second end over a spark plug 6 to connect a terminal portion of the spark plug 6 to the electric conductor 4. To avoid leaks between the inner peripheral surface of the plug cap body 2 and the outer peripheral surface of an insulator 7 of the spark plug 6, the inner diameter of the plug cap body 2 receiving the insulator 7 of the spark plug 6 is smaller than the outer diameter of the insulator 7 so that the outer peripheral surface of the insulator 7 closely contacts the inner peripheral surface of the plug cap body 2 for exhibition of an electrical sealing property. For example, when the outer diameter D1 of a major diameter portion of a corrugated portion 7a of the insulator 7 is 10.2 mm and the outer diameter D2 of a cylindrical portion 7b of the insulator 7 is 10.5 mm, the inner peripheral surface diameter A of the plug cap body 2 receiving the spark plug 6 is 9.5 mm which is smaller than the outer diameter D1 of the major diameter portion of the corrugated portion 7a. The inner diameter of the plug cap body 2 is of the same size or slightly increases gradually in a tapered configuration in a portion corresponding to the insertion position of the cylindrical portion 7b toward an insertion end at which the spark plug 6 is relatively inserted into the plug cap body 2. The inner diameter B of the plug cap body 2 at the insertion end is 9.9 mm. Recently, a need has arisen to improve the electrical sealing property with increasing voltage required for the spark plug 6. For improvement in the electrical sealing property, the plug cap body 2 has been designed to have a smaller inner diameter A to squeeze the insulator 7 of the spark plug 6 more tightly. For example, when the outer diameter D1 of the major diameter portion of the corrugated portion 7a of the insulator 7 is 10.2 mm and the outer diameter D2 of the cylindrical portion 7b of the insulator 7 is 10.5 mm, the inner peripheral surface diameter A of the plug cap body 2 receiving the spark plug 6 is 8.6 mm which is smaller than the outer diameter D1 of the major diameter portion of the corrugated portion 7a. The inner diameter of the plug cap body 2 slightly increases gradually toward the insertion end in a tapered configuration in the portion corresponding to the insertion position of the cylindrical portion 7b. The inner diameter B of the plug cap body 2 at the insertion end is 9.0 mm. The corrugated portion 7a and cylindrical portion 7b of the insulator 7 are relatively inserted into the plug cap body 2 having the inner diameters A and B which are smaller than the outer diameters D1 and D2 of the insulator 7 of the spark plug 6. This provides the improved electrical sealing property but requires a greater force to insert the insulator 7 of the spark plug 6 into the plug cap body 2. In particular, when removed from the spark plug 6, the plug cap body 2 acts as a sucking-disc and an abnormally great force is required to remove the plug cap body 2 from the spark plug 6, resulting in very low operating efficiency. To increase the sealing properties of the plug cap device is thus only a part or "one half" of the object underlying the invention, as in addition to improve electrical sealing properties it is also very desirable to improve the operating efficiency when the plug cap device is to be fitted over or removed from a spark plug, respectively, as this is also a great demand in practice. That is, there are two requirements which are seemingly contradictory. In other words, improving the sealing property by reducing the inner diameter of the plug cap body such that it fits snuggly to the outer circumference of the spark plug will deteriorate operating efficiency due to the sucking-disc effect and vice versa. The present invention according to new claim 1 suggests that the inner diameter ΦQ of the second portion of the opening of the plug cap body gradually decreases from the inner diameter ΦP of the first portion toward an insertion end from which said spark plug is inserted into the opening such that the second portion of the opening has a tapered configuration. It is thus an object of the present invention to provide a plug cap device for an internal combustion engine which ensures an improved electrical sealing property and provides high operating efficiency when fitted over and removed from a spark plug. According to the present invention there is provided a plug cap device for an internal combustion engine, comprising: a tubular plug cap body made of a flexible insulating material and having an opening; and a spark plug including a terminal portion and an insulator having a corrugated portion continuous with said terminal portion and a cylindrical portion continuous with said corrugated portion, wherein first said terminal portion, followed by said corrugated portion and said cylindrical portion are inserted in this order into said opening of said plug cap body so that the outer peripheral surface of a major diameter portion of said corrugated portion and the outer peripheral surface of said cylindrical portion come in close contact with the inner peripheral surface of said opening of said plug cap body, said opening having a first portion corresponding to the insertion position of said corrugated portion and a second portion corresponding to the insertion position of said cylindrical portion, the inner diameter of said first portion of said opening being slightly smaller than the outer diameter of the major diameter portion of said corrugated portion. This is the known plug cap device according to the above mentioned US-A-3,995,929. According to the present invention the inner diameter of said second portion of said opening gradually decreases from the inner diameter of said first portion toward an insertion end from which said spark plug is inserted into said opening of said plug cap body such that said second portion of said opening has a tapered configuration when the spark plug is not inserted. Since the inner diameter of the opening of the plug cap body in the first portion corresponding to the insertion position of the corrugated portion of the insulator is slightly smaller than the outer diameter of the major diameter portion of the corrugated portion and the inner diameter of the opening of the plug cap body in the second portion corresponding to the insertion position of the cylindrical portion of the insulator decreases gradually toward the insertion end from the inner diameter of the first portion to provide the tapered configuration, the inner peripheral surface of the opening of the plug cap body reliably contacts the outer peripheral surface of the major diameter portion of the corrugated portion of the insulator and the outer peripheral surface of the cylindrical portion of the insulator, and the electrical sealing property is insured. In particular, the inner diameter of the second portion of the opening of the plug cap body which decreases gradually toward the insertion end in the tapered configuration, ensures improved electrical sealing property in the second portion of the opening of the plug cap body, and meets the need for high voltage. The improved electrical sealing property ensured in the second portion of the opening of the plug cap body permits the inner diameter of the first portion of the plug cap body to be larger and closer to the outer diameter of the major diameter portion of the corrugated portion. This reduces the insertion and removal resistances of the serrated corrugated portion when the plug cap body is fitted over and removed from the spark plug, providing high operating efficiency at that time. These and other details, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a sectional view of a principal part of a first preferred embodiment according to the present invention; Fig. 2 is a sectional view of the principal part of the first preferred embodiment, with a spark plug fitted therein; Fig. 3 is a sectional view of a principal part of a second preferred embodiment according to the present invention; Fig. 4 is a sectional view of a principal part of a modification of the second preferred embodiment; Fig. 5 is a sectional view of a principal part of the prior art; and Fig. 6 is a sectional view of the principal part of the prior art, with a spark plug fitted therein. Referring to the drawings, a first preferred embodiment of the present invention will now be described. As shown in Figs. 1 and 2, a plug cap body 10 of a plug cap device 11 for an internal combustion engine of independent ignition type is a tubular flexible insulator made of an elastic material such as silicone rubber. An electric conductor 12 is housed and held in the plug cap body 10 in the same manner as in the prior art. A spark plug 14 includes a terminal portion 17, and an insulator 16 having a corrugated portion 16a continuous with the terminal portion 17 and a cylindrical portion 16b continuous with the corrugated portion 16a. The plug cap body 10 includes an opening 15 receiving the spark plug 14 and having an inner peripheral surface including a first portion 15a corresponding to the insertion position of the corrugated portion 16a of the insulator 16 of the spark plug 14 and a second portion 15b corresponding to the insertion position of the cylindrical portion 16b of the insulator 16 of the spark plug 14. The inner diameter P of the first portion 15a of the opening 15 corresponding to the insertion position of the corrugated portion 16a is slightly smaller than the outer diameter R1 of a major diameter portion of the corrugated portion 16a and is of the same size in the direction of relative insertion of the spark plug 14. The inner diameter Q of the second portion 15b of the opening 15 corresponding to the insertion position of the cylindrical portion 16b slightly decreases gradually toward an insertion end at which the spark plug 14 is relatively inserted into the plug cap body 10 from the inner diameter P of the first portion 15a to provide a tapered configuration. The inner peripheral surface diameter P of the first portion 15a of the plug cap body 10 corresponding to the insertion position of the corrugated portion 16a is R1 × 0.85 to R1 × 0.95 mm where R1 is the outer diameter of the major diameter portion of the corrugated portion 16a of the insulator 16. The inner diameter Q1 of the second portion 15b corresponding to the insertion position of the cylindrical portion 16b at the insertion end is R2 × 0.80 to  R2 × 0.90 mm where R2 is the outer diameter of the cylindrical portion 16b of the insulator 16. More specifically, when the outer diameter R1 of the major diameter portion of the corrugated portion 16a of the insulator 16 is 10.2 mm and the outer diameter R2 of the cylindrical portion 16b of the insulator 16 is 10.5 mm, the inner peripheral surface diameter P of the first portion 15a of the plug cap body 10 corresponding to the insertion position of the corrugated portion 16a is 9.5 mm which is slightly smaller than the outer diameter R1 of the major diameter portion of the corrugated portion 16a. The inner diameter Q1 of the second portion 15b corresponding to the insertion position of the cylindrical portion 16b at the insertion end is 8.6 mm. An end surface 15c at the insertion end of the opening 15 of the plug cap body 10 has a diameter gradually increasing in the direction of the relative insertion of the spark plug 14 to provide a tapered configuration. In the first preferred embodiment as above constructed, the terminal portion 17, corrugated portion 16a, and cylindrical portion 16b of the spark plug 14 are inserted in order into the opening 15 of the plug cap body 10 and press fitted therein, as shown in Fig. 2. The terminal portion 17 is then connected to a spring portion 12a of the electric conductor 12. At this time, the plug cap body 10 is elastically deformed so that the inner peripheral surface of the first portion 15a corresponding to the insertion position of the corrugated portion 16a comes in close contact with the outer peripheral surface of the major diameter portion of the corrugated portion 16a and the inner peripheral surface of the second portion 15b corresponding to the insertion position of the cylindrical portion 16b comes in close contact with the outer peripheral surface of the cylindrical portion 16b. This prevents leaks from the surface of the insulator 16 of the spark plug 14, insuring the electrical sealing property. In particular, since the inner diameter Q of the second portion 15b of the plug cap body 10 corresponding to the insertion position of the cylindrical portion 16b gradually decreases toward the insertion end in the tapered configuration, the plug cap device 11 ensures improved electrical sealing property in the second portion 15b of the plug cap body 10 corresponding to the insertion position of the cylindrical portion 16b, and meets the need for high voltage. The improved electrical sealing property ensured in the second portion 15b of the plug cap body 10 corresponding to the insertion position of the cylindrical portion 16b enables the inner diameter P (9.5 mm) of the first portion 15a of the plug cap body 10 corresponding to the insertion position of the corrugated portion 16a to be larger and closer to the outer diameter R1 (10.2 mm) of the major diameter portion of the corrugated portion 16a than the prior art inner diameter A (8.6 mm). Thus, the insertion resistance and removal resistance of the serrated corrugated portion 16a are reduced when the plug cap body 10 is fitted over and removed from the spark plug 14, providing high operating efficiency at that time. Furthermore, since the end surface 15c at the insertion end of the opening 15 of the plug cap body 10 is chamfered so that the diameter increases gradually in the tapered configuration, the wall thickness of the plug cap body 10 gradually decreases toward the insertion end. Such construction permits loads to gradually increase as the plug cap body 10 is fitted over the insulator 16 of the spark plug 14, providing an improved feeling during insertion and removal. If a general standard of electrical sealing property is required, the inner diameters P and Q of the opening 15 of the plug cap body 10 may be larger to reduce the insertion and removal resistances. Referring to the drawings, a second preferred embodiment according to the present invention will now be discussed. As shown in Fig. 3, a plug cap body 20 of a plug cap device 21 for an internal combustion engine is of tubular configuration and made of an insulative thermosetting resin or the like. In the same manner as in the prior art, an electric conductor 22 is housed and held in the plug cap body 20, and an ignition cable (not shown) introduced into the plug cap body 20 at its first end is connected to the electric conductor 22. The plug cap body 20 includes a first opening 20a at its second end, and a tubular cap element 23 which is a flexible insulator made of an elastic material such as silicone rubber is fitted to the second end of the plug cap body 20. A circumferential annular groove 23a is formed in a first end surface of the cap element 23, and the second end of the plug cap body 20 is fitted in the groove 23a. At this time, a circumferential ledge 24 formed on the outer peripheral surface of the plug cap body 20 at its end engages an groove 23b of the cap element 23, which is held and prevented from withdrawal from the plug cap body 20. The first opening 20a of the plug cap body 20 is continuous with a second opening 25 of the cap element 23. The inner peripheral surface of the second opening 25 of the cap element 23 receiving the spark plug 14 includes a first portion 25a corresponding to the insertion position of the corrugated portion 16a of the insulator 16 of the spark plug 14 and a second portion 25b corresponding to the insertion position of the cylindrical portion 16b of the insulator 16 of the spark plug 14. The inner diameter S of the first portion 25a of the second opening 25 corresponding to the insertion position of the corrugated portion 16a is slightly smaller than the outer diameter R1 of the major diameter portion of the corrugated portion 16a and is of the same size in the direction of relative insertion of the spark plug 14. The inner diameter T of the second portion 25b of the second opening 25 corresponding to the insertion position of the cylindrical portion 16b decreases gradually toward the insertion end at which the spark plug 14 is relatively inserted into the plug cap body 20 from the inner diameter S of the first portion 25a to provide a tapered configuration. For example, the inner peripheral surface diameter S of the first portion 25a of the cap element 23 corresponding to the insertion position of the corrugated portion 16a is R1 × 0.85 to R1 × 0.95 mm where R1 is the outer diameter of the major diameter portion of the corrugated portion 16a of the insulator 16 (for example, 10.2 mm). The inner diameter T1 of the second portion 25b corresponding to the insertion position of the cylindrical portion 16b at the insertion end is R2 × 0.80 to R2 × 0.90 mm where R2 is the outer diameter of the cylindrical portion 16b of the insulator 16 (for example, 10.5 mm). In the second preferred embodiment as above constructed, when the plug cap body 20 is fitted over the spark plug 14 in substantially the same manner as the first preferred embodiment, the terminal portion 17 of the spark plug 14 is inserted into the first opening 20a through the second opening 25 and is then connected to the electric conductor 22. At this time, the cap element 23 is elastically deformed so that the inner peripheral surface of the first portion 25a corresponding to the insertion position of the corrugated portion 16a comes in close contact with the outer peripheral surface of the major diameter portion of the corrugated portion 16a and the inner peripheral surface of the second portion 25b corresponding to the insertion position of the cylindrical portion 16b comes in close contact with the outer peripheral surface of the cylindrical portion 16b. This prevents leaks from the surface of the insulator 16 of the spark plug 14, ensuring the electrical sealing property. In particular, since the inner diameter T of the second portion 25b of the cap element 23 corresponding to the insertion position of the cylindrical portion 16b decreases gradually toward the insertion end in the tapered configuration, the plug cap device 21 ensures improved electrical sealing property in the second portion 25b of the cap element 23 corresponding to the insertion position of the cylindrical portion 16b, and meets the need for high voltage. The improved electrical sealing property ensured in the second portion 25b of the cap element 23 corresponding to the insertion position of the cylindrical portion 16b enables the inner diameter S of the first portion 25a of the cap element 23 corresponding to the insertion position of the corrugated portion 16a to be larger and closer to the outer diameter R1 (10.2 mm) of the major diameter portion of the corrugated portion 16a than the prior art inner diameter A (8.6 mm). Thus, the insertion resistance and removal resistance of the serrated corrugated portion 16a are reduced when the plug cap body 20 is fitted over and removed from the spark plug 14, providing high operating efficiency at that time. As shown in Fig. 4, an end surface 25c at the insertion end of the second opening 25 of the cap element 23 may have a diameter gradually increasing in the direction of the relative insertion of the spark plug 14 to provide a tapered configuration in the same manner as the first preferred embodiment. If a general standard of electrical sealing property is required, the inner diameters S and T of the second opening 25 of the cap element 23 may be larger to reduce the insertion and removal resistances. The present invention is not limited to the plug cap devices 11 and 21 of ignition type of the first and second preferred embodiments but may be applied to any types of plug cap devices. While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention as defined in the appended claims.

Claims: A plug cap device for an internal combustion engine, comprising: a tubular plug cap body (10) made of a flexible insulating material and having an opening (15); and a spark plug (14) including a terminal portion (17) and an insulator (16) having a corrugated portion (16a) continuous with said terminal portion (17) and a cylindrical portion (16b) continuous with said corrugated portion (16a), wherein first said terminal portion (17), followed by said corrugated portion (16a) and said cylindrical portion (16b) are inserted in this order into said opening (15) of said plug cap body (10) so that the outer peripheral surface of a major diameter portion of said corrugated portion (16a) and the outer peripheral surface of said cylindrical portion (16b) come in close contact with the inner peripheral surface of said opening (15) of said plug cap body (10), said opening (15) having a first portion (15a) corresponding to the insertion position of said corrugated portion (16a) and a second portion (15b) corresponding to the insertion position of said cylindrical portion (i6b), the inner diameter (P) of said first portion (15a) of said opening (15) being slightly smaller than the outer diameter (R1) of the major diameter portion of said corrugated portion (16a), characterized in that the inner diameter (Q) of said second portion (15b) of said opening (15) gradually decreases from the inner diameter (P) of said first portion (15a) toward an insertion end from which said spark plug (14) is inserted into said opening (15) of said plug cap body (10) such that said second portion (15b) of said opening (15) has a tapered configuration when the spark plug (14) is not inserted. The plug cap device of claim 1, wherein said plug cap body (10) has an end surface (15c) at said insertion end of said opening (15), and said end surface (15c) has a diameter gradually increasing in the direction of insertion of said spark plug (14) to provide a tapered configuration. The plug cap device of claim 1, wherein said plug cap body (10) is made of silicone rubber. The plug cap device of claim 1 or 2, wherein said tubular plug cap body (20) has a first opening (20a) and a tubular cap element (23) is provided made of a flexible insulating material and having a second opening (25), said cap element (23) being fitted to a second end of said plug cap body (20) so that said second opening (25) is continuous with said first opening (20a), said terminal portion (17) of said spark plug (14) being inserted into said first opening (20a) through said second opening (25) and the inner diameter (T) of a second portion (25b) of said second opening (25) corresponding to the insertion position of said cylindrical portion (16b) of said spark plug (14) gradually decreases from the inner diameter (S) of a first portion (25a) corresponding to an insertion position of said corrugated portion (16a) of said spark plug (14) toward an insertion end from which said spark plug (14) is inserted into said plug cap body (20) to provide a tapered configuration. The plug cap device of claim 4, wherein said cap element (23) has an end surface (25c) at said insertion end of said second opening (25), and said end surface (25c) has a diameter gradually increasing in the direction of insertion of said spark plug (14) to provide a tapered configuration. The plug cap device of anyone of claims 1 to 5, wherein said plug cap body (20) is made of a thermosetting resin. The plug cap device of claim 4 or 5, wherein said cap element (23) is made of silicone rubber.

Inventors: IMANISHI HISAO, IMANISHI, HISAO
Applicants: SUMITOMO WIRING SYSTEMS, SUMITOMO WIRING SYSTEMS, LTD.
Application year: 1994
Main classification: H01T13
IPC codes: H01T13